[GXT 2.3.1a] In RowEditor#stopEditing ComboBoxes are invalid

After migrating from GXT 2.2.5 to 2.3.1a I've got a problem with RowEditors and ComboBoxes.

When I'm using a ComboBox that doesn't allow empty values and I'm clicking 'Save' in the RowEditor the results will not be saved. After debugging I noticed in the RowEditor#stopEditing method, that ComboBoxes will be cleared (line 503) and I think because of this, the following isValid-check for the RowEditor returns false (ComboBox is empty -> not allowed) and editing is canceled.

Is there a special reason why the ComboBoxes will be cleared? In 2.2.5 I can't find this and everything there is working great.

Thanks for your help!

EDIT:
In release notes for 2.3.1 I found this bug-fix. I think I could be related to this problem:

[EXTGWT-3233] - RowEditor Grid - Blank combo value after first edit

EDIT2:
Here is an example with EntryPoint. Compile and open it, than change a row and click save - nothing will be saved.
If you replace the RowEditor with the FixedRowEditor everything works fine. The FixedRowEditor uses the same stopEditing-method than the RowEditor - except the ComboBox#clear() call. So this call should be the problem.